A feedwater heater for an hrsg is provided with a monitoring unit for detecting the presence of condensation in the feedwater heater. The monitoring unit includes a dielectric band around one of the tubes of the feedwater heater near the location where the feedwater is directed into the heater and a conductive band located around the dielectric band. The unit also includes a conductivity sensor installed between a ground on the feedwater heater and the conductive element. Hot gases containing moisture pass through the feedwater heater, and if the temperature of surfaces in the region of the tube around which the dielectric and conductive bands extend drops below the dew point of the gas, an electrically conductive condensate will appear those surfaces and on the tube and will flow over the dielectric band, completing an electric circuit between the tube and the conductive band. The conductivity sensor detects this and hence detects the presence of the condensation.
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11. A process for detecting electrically conductive condensate in a feedwater heater, said process comprising:
installing a dielectric element on a surface of the feedwater heater near the location where condensation is likely to occur;
installing a conductive element over the dielectric element such that the dielectric element normally isolates the conductive element from the surface;
monitoring electrical conductivity between the surface and the conductive element.
3. In combination with an hrsg including
a duct through which a gas having a dew point flows, with the gas being capable of producing a condensate that is electrically conductive;
a superheater located in the duct;
an evaporator located in the duct downstream from the superheater;
and a feedwater heater located in the duct downstream from the evaporator;
a monitoring system comprising:
a surface on the feedwater heater over which the condensate will flow;
a dielectric element located over the surface;
a conductive element located over the dielectric element and normally being isolated from the surface by the dielectric element; and
a monitoring device for detecting the presence of an electrical circuit between the feedwater heater and the conductive element.
1. The combination comprising:
a heat exchanger into which liquid water flows and through which a hot gas having a dew point passes so that heat transfers from the gas to the water, the heat exchanger being formed from an electrically conductive material and having a surface along which a conductive condensate will form if the temperature of the surface falls below the dew point of the gas;
a dielectric element adjacent to the surface;
a conductive element over the dielectric element and normally being electrically isolated from the heat exchanger by the dielectric element; and
a monitoring device for detecting electrical conductivity between the heat exchanger and the conductive element;
whereby when a conductive condensate flows over the dielectric element and bridges the space between the surface on the heat exchanger and the conductive element, the monitoring device will sense electrical conductivity through the condensate and hence the presence of the condensate.
13. The combination comprising:
a heat exchanger through which a gas having a dew point passes, the heat exchanger being formed from an electrically conductive material and having a tube that extends substantially vertically and has a surface along which a conductive condensate will form if the temperature of the surface falls below the dew point of the gas;
a dielectric element adjacent to and below the surface on the tube;
a conductive element below the surface on the tube and over the dielectric element and normally being electrically isolated from the heat exchanger by the dielectric element; and
a monitoring device for detecting electrical conductivity between the heat exchanger and the conductive element;
whereby when a conductive condensate flows over the dielectric element and bridges the space between the surface on the heat exchanger and the conductive element, the monitoring device will sense electrical conductivity through the condensate and hence the presence of the condensate.
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This application derives and claims priority from U.S. provisional application 60/557,626 filed Mar. 30, 2004.
Not applicable.
This invention relates in general to heat exchangers and, more particularly, to a process and apparatus for detecting condensation in a heat exchanger.
Natural gas represents a significant source of electrical energy in the United States. It burns with few emissions, and is available throughout much of the country. Moreover, the plants which convert it into electrical energy are efficient and, in comparison to hydroelectric projects and coal-fired plants, they are relatively easy and inexpensive to construct. In the typical plant, the natural gas burns in a gas turbine which powers an electrical generator. The exhaust gases—essentially carbon dioxide and steam—leave the gas turbine at about 1200° F. (649° C.) and themselves represent a significant source of energy. To harness this energy, the typical combined cycle, gas-fired, power plant also has a heat recovery steam generator (HRSG) through which the hot exhaust gases pass to produce steam which powers a steam turbine which, in turn, powers another electrical generator. The exhaust gases leave the HRSG at temperatures on the order of 150° F. (66° C.).
The HRSG basically comprises a series of heat exchanges housed in a duct. Water which is derived from condensing steam discharged from the steam turbine enters the HRSG at a feedwater heater where it undergoes a rise in temperature. The higher temperature water then flows into an evaporator where it is converted into steam, most if not all saturated steam. That steam flows into a superheater which converts it into superheated steam, and the superheated steam flows on to the steam turbine to power it. The hot gases derived from the combustion flow in the opposite direction, encountering the superheater, then the evaporator, and finally the feedwater heater.
Thus, the gases are at their coolest temperatures in the region of the feedwater heater and beyond. Natural gas contains traces of sulfur, and during the combustion the sulfur combines with oxygen to produce oxides of sulfur. Moreover, the combustion produces ample quantities of water in the form of steam. If the exhaust gases remain above the dew point for the gases, which is about 107° F. (42° C.), the oxides of sulfur pass out of the HRSG and into a flue. However, the low temperature feedwater has the capacity to bring the tubes at the downstream end of the feedwater heater below the dew point of the water in the exhaust gases, and when this occurs, water condenses on tubes. The oxides of sulfur in the flue gas unite with that water to form sulfuric acid which is highly corrosive. Other acids may likewise form.
In order to deter the formation of acids, operators of HRSGs control the temperature of the water entering the feedwater heater, so that it remains well above the dew point for the gases. This assures that no condensation occurs in the feedwater heater. And to be safe, the temperature of the entering water needs to be high, because the dew point temperature of the gases is difficult to predict in that it is a function of several parameters. If the temperature of the entering water could be lowered, the water would extract more energy from the gases, and they would pass beyond the feedwater heater at a lower temperature.
The problem of condensation in feedwater heaters or economizers is not confined solely to HRSGs installed downstream from gas turbines. Indeed, it can occur almost anywhere energy is extracted from hot gases flowing though a duct to heat the feedwater for a boiler. For example, many power plants convert the hot gases derived from the combustion of fossil fuels, such as coal or oil, directly into steam, and the boilers required for the conversion, to operate efficiently, should have feedwater heaters—heaters which should not produce condensation. Also, systems exist for producing steam from the hot gases derived from the incineration of waste, and they likewise have boilers including feedwater heaters that should not be subjected to condensation.
Referring now to the drawings a heat recovery steam generator (HRSG) A (
The HRSG A includes a duct 2 having an inlet end 4 and a discharge end 6 which leads into a stack or flue. Hot gases derived from the combustion of natural gas or some other fuel enter the duct 2 at the inlet end 4, pass through it, and leave at the discharge end 6. The gases contain carbon dioxide and steam and trace mounts of compounds which if united with liquid water can form corrosive substances such as acids.
In addition to the duct 2, the HRSG includes several heat exchangers that are housed in succession within the duct 2 (
The operator of the HRSG A maintains a measure of control over the temperature of the feedwater that enters the feedwater heater 14. Preferably, that temperature should be low to extract maximum heat from the gases flowing through the duct 2, yet it should remain above the dew point of the gases to avoid condensation from developing in the feedwater heater 14. The monitoring unit B enables the operator of the HRSG to achieve these objectives.
The feedwater heater 14 includes (
The monitoring unit B basically comprises (
The actuating terminal 42 includes (
In the operation of the HRSG A, hot gases, the products of combustion of a fuel, such as natural gas, enter the duct 2 at its inlet end 4. Here the gases exist at an extremely high temperature on the order of 1200° F. (649° C.). The gases pass through the superheater 10 where heat is extracted from them and then through the evaporator 12 where more heat is extracted. The temperature of the gases drops appreciably. When the gases encounter the feedwater heater 14 the temperature may have dropped to between 300° F. (149° C.) and 200° F. (93° C.). The dew point for the gases, although difficult to predict, is on the order of 107° F. (42° C.), so the surfaces of the feedwater heater 14 should remain above the dew point. Yet the feedwater 14 should maintain the surfaces of the feedwater heater 14 at a temperature only slightly above the dew point of the gases, perhaps 5° F. (2.8° C.) above the dew point. This enables the HRSG A to extract the maximum amount of heat from the gases without producing condensation and the corrosion that it causes. And the operator of the HRSG A does maintain a measure of control over the temperature of the water that enters the feedwater heater 14.
Thus, to insure that the HRSG A operates most efficiently, the operator reduces the temperature of the feedwater while monitoring the conductivity meter 44. As long as no condensation develops on the header 20 or the nearby regions of the tubes 24, the conductivity meter 44 will not register an alarm or other signal. However, should the feedwater cool the header 20 and nearby regions of the tubes 24 to a temperature below the dew point of the gases, the moisture in the gases will condense on the header 20 and on the bare surface 34 of the one tube 24 and will flow downwardly over the upper margin of the dielectric band 50 and along the surface of the band 50 to the conductive band 52. It completes an electrical circuit between the bare section 34 of the one tube 24 and the conductive band 52. The conductivity meter 44 registers the completion of the circuit, thereby notifying the operator of the HRSG A that the temperature of the feedwater is too low. The operator can adjust the temperature of the feedwater upwardly in increments until the conductivity meter 44 no longer registers the presence of a circuit. This of course denotes the absence of a condensate.
Variations are possible. For example, the activating terminal 42 need not be on a tube 24, but may be on some other surface, such as the side of the header 20, where condensation will also occur. Irrespective of the location of the actuating terminal 42, its dielectric and conductive elements need not extend completely around the surface on which it is mounted. Moreover, the HRSG A is depicted in its simplest form. It may include additional superheaters, evaporators and even feedwater heaters. The monitoring unit B may be used on heat exchanges other than feedwater heaters in HRSGs. Any instrument or sensor capable of detecting conductivity will suffice for the conductive meter 44. Also, the monitoring unit B may be installed on an evaporator, such as the evaporator 12. Should the unit B, when so installed, detect condensate, the operator can raise the evaporator boiling temperature.
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Oct 13 2004 | Nooter Eriksen, Inc. | (assignment on the face of the patent) | / |
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